1. Field of the Invention
The present invention relates to a method for coating cylindrical rolls. More particularly, the present invention relates to the coating of rolls with layers of two or more coating materials wherein there is no interface between the layers.
2. Description of Related Art
A variety of elastomeric coverings are used to coat rolls in applications in which such rolls require protection. Polyurethane elastomers are used as roll coatings in applications where abrasion and tear resistance, good load bearing characteristics, high hardness, and solvent resistance are required.
In steel mill applications, for example, large numbers of these rolls are used for guiding, tensioning, and otherwise engaging steel strip during rolling and pickling operations. The rolls are subjected to the strong chemical agents used for pickling and cleaning the strip. Similarly, in paper mills, polyurethane coated metal rolls are used for supporting and conveying paper through high pressure nips or paper making machine assemblies, such as, press rolls, calender stacks, and the like. The rolls used in paper mill operations require a protective cover whose thickness can be varied depending on the roll's anticipated use.
Different methods have been developed for applying coatings to metal rolls.
U.S. Pat. No. 3,520,747 discloses a process for making a structural member for use in hydrofluoric and other corrosive environments, said structural member including a base structure. The process includes the steps of roughening the base structure, applying a coating to the roughened surface of a resin selected from the group consisting of polyester, phenolic and epoxy resins, depositing a layer of glass fiber material on the resin, coating an additional quantity of the resin on the fiber glass material, which is worked into the interstices of the fiber glass material to impregnate it as well as to leave a coating of the resin material thereover, applying a layer of fibrous material selected from the group consisting of crocidilite, asbestos, and fibrous polyvinyl chloride-acrylonitrile copolymer to the second-mentioned resin coating, applying a third quantity of the resin to the surface of the fibrous material and working it into the interstices of the fibrous material to impregnate it and to leave a coating of the resin thereover.
U.S. Pat. No. 3,646,651 discloses structural materials, especially rolls for use in steel-fabricating processes, for use in corrosive and erosive environments. In one arrangement, the roll includes a base member or substrate to which is adhered a resin coating. A reinforcing mat or cloth is adhered to and partially embedded in the resin coating, before the latter hardens. A second resin layer is coated upon and partially impregnates the reinforcing mat. A second mat is then adhered to the second resin layer in a similar manner followed by a third coating of the resin material. A greater or lesser number of resin layers and reinforcing mats can be employed as required. The specific materials employed for the resin or plastic layers and the reinforcing cloths or mats can be varied according to the specific environment in which the roll will be used. For example, in the presence of hydrofluoric acid the reinforcing mat may be Dynel cloth. For resilient contact with the work a suitable elastomeric layer is placed over the last resin or plastic coating.
U.S. Pat. No. 4,368,568 discloses a polyurethane covered roll comprising a metallic roll core, a reinforcing layer adhering to the outer surface of the roll core, and formed by a nonwoven fabric impregnated with a mixture of a thermosetting resin and a fine inorganic powder, and a layer of polyurethane rubber united integrally with the outer surface of the reinforcing layer. The roll is said to be useful when used, for example, in a paper machine.
U.S. Pat. No. 4,571,798 discloses a urethane covered metal press roll provided by spray coating the roll body with a quick jelling resin formulation creating isolated small pores giving the cover a stone-like texture. The coating is applied to a desired depth by successive passes of a spray jet over the rotating roll body. The resin quickly jells and after being cured, the cover is ground to a desired diameter and finish.
Ruprecht et al., Polyurethane World Congress 1991 (Sep. 24-26, 1991) 478-481, describe rotational casting techniques useful for producing roll coverings using fast-reacting polyurethane elastomer systems. In these systems, the polyurethane reaction mixture is metered through a movable mixing head that travels at constant speed in the axial direction along the rotating roll core, a short distance above its surface. The polyurethane reaction mixture solidifies very quickly, in a matter of seconds, to produce a polyurethane coating with a thickness buildup of 4 to 5 mm. Additional layers of the polyurethane reaction mixture are applied until the desired thickness of polyurethane coating is achieved. The interval between applications is kept under 30 minutes in order to ensure good adhesion of one layer to another. After a short curing time at ambient temperature the covering is machined to the desired final thickness. The cold-curing two-component system consists of a highly reactive polyol blend and a prepolymer based on MDI yielding elastomers in a hardness range of 73 to 95 Shore A with good mechanical properties and chemical resistance. It is reported that rotational casting offers a number of advantages over conventional methods. No mould and no release agent is needed. The whole operation takes place at room temperature. Layers of different hardness and other physical properties can be combined in one covering. The overall production time is very short. Rolls and other rotationally symmetrical objects with a large difference in size and shape can be covered by one piece of equipment. Rolls with coverings made by rotational casting are successfully used in the steel industry, above all in sheet metal finishing, e.g., as backup, deflection, and squeeze rolls. Further applications include various types of rolls in paper machines and conveyer belt rolls.
The disclosures of the above-described references are incorporated herein by reference in their entirety.